How to use file_save_upload when in a tabbed form

In a vertical tab form the path to a form input is$form[‘vertical_tabs_card_details’][‘card_image’] = array( the documentation uses a simple key value for the file_save_upload function as in file_save_upload (‘card_image’); which does not work for the tabbed field nor doesfile_save_upload(“‘vertical_tabs_card_details’][‘card_image'”); or any other values i have[…]